Transistors characteristic and electrical stability of zinc tin oxide thin film transistors by solution process

Abstract

In this dissertation we fabricate solution-processed Zinc Tin Oxide thin film transistors (ZTO TFTs) First the mixed metal oxides semiconductors (M1M2O) which have heavy metal cations (M1 and M2) are primarily composed of spatially spread metal (n ? 1)d10ns0 (n ? 4) orbital with isotropic shape the magnitude of this overlap among the adjacent metal ns orbital of spherical symmetry resulted in efficient path of electron transport even if distorted amorphous structure In addition among them ZTO TFT that is indium- and gallium- free has gained an advantage because of the low-cost competition and non-toxic Second this work suggested spin-coating/solution-processing method allow for a continuous process which is advantageous for achieving simple low-cost and high-throughput production without the need for costly high-vacuum equipments Hence this thesis provided more substantial academic value Furthermore this thesis is divided into three aspects for research In the first of aspect zinc acetate tin chloride (solute) and ethylene glycol monomethyl ether (solvent) were used as precursors for preparing ZTO semiconductor films We examine the fundamental studies of ZTO precursor composition (Sn/(Zn+Sn) ratio) effects at the beginning in order to figure out the role of each component in ZTO oxide semiconductors with their material characteristic Next ultra-thin zinc-tin oxide (ZTO) films (~7 nm thick) with different Sn/(Sn+Zn) molar ratios fabricated by using a solution process in combination of spin coating method are applied as channel layers in thin film transistors (TFTs) with a bottom-gate top-contact structure With regard to material characteristics oxygen deficiency in ZTO films can be substantially decreased with the addition of different Sn contents The non-Sn added ZnO TFT cannot be turned on whereas the Sn added ZTO channel layers perform TFT characteristics adequately indicating the necessity of reducing oxygen deficiency by introducing Sn during the solution synthesizing process In the mechanism we can control the oxygen deficiency of the ZTO TFTs using various molar ratios of Sn because of a high ionic potential With a Sn/(Sn+Zn) molar ratio of 0 5 the ZTO TFT exhibits the best field-effect mobility of ~ 2 0 cm^2/Vs and a large on/off current ratio of ~10^8 The electrical characteristics of ZTO TFTs are explored and correlated to the levels of oxygen deficiency associated with various Sn contents (detailed in Chapter 3) In second aspect solution-processed ZTO films are fabricated using a volatile metal nitrate precursor species with a low annealing at 350 oC However this section of ZTO films at annealing temperature remained 500 oC Next chapter of 5 ZTO TFTs will do a series of stability study at annealing temperature of 350 oC In this chapter an ultra-thin (5 nm-thick) unpassivated zinc tin oxide (ZTO) thin-film transistor TFT fabricated with solution process exhibits a good field-effect mobility (13~14 cm^2/Vs) small subthreshold swing (~0 30 V/dec ) and high on/off current ratio (~10^8) The field-effect mobility can be further enhanced by increasing the ZTO thickness to 12 nm (~22 cm^2/Vs) and 22 nm (~29 cm^2/Vs) Furthermore ID-VG characteristics of the 5nm-thick ZTO TFT remain unaffected regardless of working in air (60% relative humidity) vacuum (3 3×10^-5 torr) or dry O2 (760 torr) atmosphere The dissimilar TFT characteristics are discussed in terms of oxygen deficiency content as well as the Fermi level position (EF to EC) for ZTO of various thicknesses to explain the moisture immunity of the 5 nm-thick solution-processed ZTO TFT (detailed in Chapter 4) In the third of aspect in practical application switching TFTs are continuously exposed to bias and/or illumination from the backlight In this section the interactions between these (such as simultaneous illumination/bias stressing) will also be discussed Solution-processed ultra-thin (3 nm) zinc tin oxide (ZTO) thin film transistors (TFTs) with a mobility of 8 cm^2/Vs are obtained with post spin-coating annealing at only 350 oC The effect of light illumination (at wavelengths of 405 nm or 532 nm) on the stability of TFT transfer characteristics under various gate bias stress conditions (zero positive negative) is investigated It is found that the ΔVth (Vth= stress 3400s - stress 0s) window is significantly positive when ZTO TFTs are under positive bias stress (PBS ΔVth= 9 98 V) and positive bias illumination stress (PBIS λ=405 nm ΔVth= 6 96 V) but ΔVth is slightly negative under only light illumination stress (IS λ= 405 nm ΔVth= -2 02 V) or negative bias stress (NBS ΔVth= -2 27 V) However the ΔVth of ZTO TFT under negative bias illumination stress is substantial and it will efficiently recover the ΔVth caused by PBS The result is attributed to the photo-ionization and subsequent transition of electronic states of oxygen vacancies (i e Vo Vo+ and Vo++) in ZTO A detailed mechanism is discussed to better understand the bias stress stability of solution processed ZTO TFTs (detailed in Chapter 5)

Date of Award	2015 Aug 26
Original language	English
Supervisor	Jen-Sue Chen (Supervisor)